1. Field
The present disclosure relates to an air foil bearing for supporting a rotating body in a rapid rotating machine, and more particularly, to an air foil bearing having a pressure dam which may concentrate and stabilize an air layer between a rotary shaft and a foil.
2. Description of the Related Art
Due to the increase of a rotation speed in various machines, a bearing supporting the rotating body is demanded to have stability and abrasion resistance at a high speed.
An air foil bearing is one of bearings developed to a rotating body with an ultrahigh speed in various machines.
The foil having a film shape supports a load of a rotary shaft, which rotates at a high speed, by using a hydrodynamic characteristic of air serving as a lubricating medium. The high-speed rotating body may be an auxiliary power device or an air conditioning system for an airplane. The air foil bearing has a configuration similar to a general air bearing, but a thin foil is inserted between the rotary shaft and the bearing housing to give additional stiffness and damping.
FIG. 1 shows a general air foil bearing.
As shown in FIG. 1, the air foil bearing includes a plurality of foil layers around a rotary shaft 4a inserted into a bearing housing 1a having a shaft hole 5a formed therein.
According to the example depicted in FIG. 1, the foil layer includes a top foil 3a and a bump foil 2a arranged from a position close to the rotary shaft 4a. Through not shown in the figure, a seam foil may be disposed between the bump foil 2a and the bearing housing 1a. 
One end of each of the foils 2a, 3a is fixed to the inner side of the bearing housing 1a by a pin, and the other end extends approximately along the shape of the inner surface of the housing 1a to form a free end. The surfaces of the foils 2a, 3a are coated to enhance the friction.
The top foil 3a supports the rotary shaft 4a and is disposed so that an air lubrication film is interposed between the top foil 3a and the rotary shaft 4a. The bump foil 2a has high rigidity and is installed to improve the load supporting ability of the rotary shaft 4a, and if a hydrodynamic pressure is generated by the rotation of the rotary shaft 4a, the bump foil 2a is deformed in the circumferential direction to support the load.
If the rotary shaft 4a rotates at a high speed, a hydrodynamic pressure is generated by an air layer formed between the top foil 3a and the rotary shaft 4a, and the rotary shaft 4a rotates while receiving a floating force on the inner surface of the top foil 3a by the generated hydrodynamic pressure.
The foils described above have a function of damping vibration generated when the rotary shaft 4a rotates inside the air foil bearing. In other words, the energy generated when the rotary shaft is vibrating is dispersed by the elasticity of each foil and the coulomb friction generated when the foils relatively move in the circumferential direction while closely adhering to each other by a hydrodynamic pressure applied at the high-speed rotation of the rotary shaft, thereby damping the vibration.
In the air foil bearing, the hydrodynamic pressure of the air layer generated between the rotary shaft 4a and the top foil 3a is an important factor which dominates a vibration damping force and a load supporting force of the bearing.
As shown in FIG. 1, when the rotary shaft 4a rotates at a high speed in the clockwise direction, the hydrodynamic pressure of the air layer formed between the rotary shaft 4a and the top foil 3a is concentrated by the rotation force of the rotary shaft 4a or the like to a hydrodynamic pressure concentration region which corresponds to a lower region of ±60° based on a vertical axis (y axis), when the bearing is observed at the front, and a hydrodynamic pressure maximum point (Pmax) at which the hydrodynamic pressure is maximized is formed in the hydrodynamic pressure concentration region.
In the existing technique, since the air layer between the rotary shaft 4a and the top foil 3a is not effectively concentrated, the bearing does not endure a sufficient load at high-speed rotation and thus has deteriorated hydrodynamic safety.